1. Field of the Invention
The present invention relates to a welding method and welding apparatus for joining a resin member having a laser-transmitting property and a resin member having laser absorptiveness, by using a laser beam.
2. Description of the Related Art
A technique of superimposing a resin member having a transmitting property with respect to a laser beam of a predetermined wavelength and a resin member having absorptiveness with respect to a laser beam of the same wavelength and casting a laser beam from the side of the laser-transmitting resin member to deposit the two resin members, is well known as described in JP-A-2002-337236 and so on.
This technique will now be briefly described. The laser beam passes through the laser-transmitting resin almost without being absorbed at all and is absorbed in the vicinity of the surface of the laser-absorptive resin member. The energy of the absorbed laser beam is converted to heat, which heats the surface of the laser-absorptive resin member. Also the vicinity of the surface of the resin of the laser-transmitting member resin in contact with the surface of the laser-absorptive resin is heated by heat transfer. As a result, a melted layer is formed on the contact surface between the laser-transmitting resin member and the laser-absorptive resin member, and the two resin members are welded. In the welding, in order to secure adhesion of the laser-transmitting resin member and the laser-absorptive resin member, a load is applied in the direction of the superimposition and the laser beam is cast in a state where the resin members are pressurized on the welding surface. Therefore, a part of the melted layer is discharged outward and the welded members approach each other more closely than before the welding.
In the traditional laser welding method, the approaching distance of the two resin members varies with respect to the irradiation time of a predetermined laser beam. Therefore, the dimensional accuracy of the welded unit is lowered. This variance is caused by various factors. These include instability of laser power, difference in transmittance between individual laser-transmitting resins, and so on. On the other hand, the method proposed in JP-A-2002-337236 is a management method in which welding is ended when a predetermined approaching distance is reached, and therefore the variance in the approaching distance is reduced.
However, the approaching motion proceeds even during a delay time in terms of control from the time when it is detected that the predetermined approaching distance is reached until the welding is stopped. Since the approaching distance during this period varies, there still is variance in the approaching distance.
To secure the dimensional accuracy of the welded unit, it is most effective to employ a structure in which two resin members being welded are butted against each other at other parts than the welded part. If the structure is designed to realize predetermined positional accuracy at the position where the two resin members are butted against each other, the influence of variance in the approaching distance in the welding can be eliminated. Of course, in this case, the time until the two resin members are butted against each other varies among individual units. In order for resin members to be securely butted against each other in all individual units, it is necessary to set a longer laser beam casting time than a variance range. Therefore, for a unit which takes a short time for the resin members to be butted against each other, the laser beam casting time after the butting is excessively long, and a problem of lowering in the joining strength at the welded part arises. The lowering in the strength is caused by the following factors. Since the welded part is pressurized until the two resin members are butted against each other, the melted resin is sequentially discharged outward and the melted layer is prevented from being excessively heated. However, after the butting, since the butted part receives loading, the pressurizing force does not act on the welded part. Consequently, the discharge of the melted layer stops. If irradiation with the laser beam is further continued in this state, the temperature of the melted layer rises, which causes generation of voids and thermal decomposition. Thus, the joining strength is lowered.